Help Pls DWC brown stuff

Hello there i have a dwc of 12 lt running for 2 weeks. I use tap water and i just started the AN grow program, at quarter strenght per lt. My temps and humidity are in the normal range, the water temp is around 22 -23 celcius and ph ranges from 6.2 to 7 which i control it with ph down . Yesterday i changed the nutes and i saw change in the colour of the roots. Pls any help will be appreciated, here are some photos, the top of the plant looks gr8. Also i didn't notice any weird smells just those of the AN nutrient. Thnx a lot

Does it wipe off? Could just be algae.
Temp should be ok. Are you using any beneficial bacterias or a sterile res?

Also ph for hydro does better at 5.2-6.2.

No it does not wipe off, yes i use voodoo juice, piranha and tarantula, and yes the res is sterilized with alcohol before i change the water, got 2 buckets and i change every week in turns. The previous day I always rub with alcohol and then rinse with water. Thnx a lot for the reply

Probably just stain. Bring your to 5.5-6.5 though.

Definitely drop ur pH then see what happens

First of all thnx a lot for the replies, went to the grow shop yesterday and the guy told also its is probably a stain due to the increase of nutes. I changed the water with much less nutes, and i'll monitor for the next couple of days. He told me if it doesn't lose this colour to use oxyplus. Thnx again guys.